Magnetic resonance imaging (MRI) is now coming into wide-spread commercial usage. Nevertheless, there are still many possible areas for improvement. For example, desired improvements are still sought to improve the signal-to-noise ratio in NMR responses and, accordingly, in resulting NMR images.
One presiously known technique for improving the attainable signal-to-noise ratio involves the use of quadrature RF transmit/receive coils. For example, a general description of such quadrature RF coils and of the potential benefits to be derived from use of same is provided in the following prior publications:
"Quadrature Detection Coils--A Further .sqroot.2 Improvement in Sensitivity" by Chen et al, J. Mag. Res. 54, 324-327 (1983)
"Quadrature Detection in the Laboratory Frame" by Hoult et al, Mag. Res. Med. 1 339-353 (1984)
"A Quadrature Probe For Adult Head NMR Imaging" by Sank et al, Department of Radiology, NIH, Proceedings of the Third Annual Meeting of the Society of Magnetic Resonance in Medicine, New York, August 1984, pp 650-651
"Radio Frequency Penetration Effects in MR Imaging: Simulation/Experiment with Linearly Polarized and Circularly Polarized RF Fields" by Glover et al, GE Medical Systems, Proceedings of the Third Annual Meeting of the Society of Magnetic Resonance in Medicine, New York, August 1984, pp 264-265.
(The above-referenced Arakawa et al pending application Ser. No. 827,609 and Fehn et al (UC 86-044-1) also specifically relate to quadrature detection RF coils for MRI.)
These prior publications describe a pair of matched quadrature RF coils wherein each coil includes four axially extending legs disposed at 60.degree., 120.degree., 60.degree. and 120.degree. intervals about a common cylinder--with one coil being rotated by 90.degree. spatially with respect to the other. (It is possible that some other reported QD coils use axial legs disposed at equal 90.degree. intervals.) Quadrature phase outputs from the two separate coils are subsequently combined in a 90.degree. hybrid so as to produce an output having increased signal-to-noise ratio (e.g., because the non-coherent noise will tend to cancel when the two signals are coherently added with an appropriate 90.degree. phase shift in one of them). As is also noted in these references, one or both of the coils may be advantageously utilized for transmitting RF NMR excitation pulses into the enclosed volume to be imaged so as to further enhance the RF field uniformity and/or so as to reduce the required level of transmitted RF power.
Ideally, there should be no inductive coupling between the two RF quadrature coils. In reality, there is always inherently some spurious coupling. Nevertheless, the effective isolation between the two coils can be improved by purposefully adding some additional coupling between the coils of the proper amplitude and phase to cancel (or at least substantially reduce) the unwanted but inherent spurious inter-coil coupling. It is apparently for this purpose that the prior art has employed conductive areas (termed "paddles") between some sections of the coil legs.
Although the general theory of quadrature detection coils is known in the prior art, the successful realization of a commerically reproducible working embodiment of such a system with minimum coupling between the coils, an RF balanced coil structure--and one which is nevertheless configured spatially in a manner which facilitates not only manufacture but also actual use--remains as a difficult task.
Some prior approaches have employed coil structures having both ends of the coil structure open-ended and with axially extending feedlines emanating from opposite ends of the overall structure and respectively associated with each of the two coils.
The earlier-referenced related Arakawa et al application Ser. No. 827,609 discloses an improved and novel structure for quadrature RF coil arrangement in an MRI apparatus which provides very good isolation between the two coils even though only one end of the coil is open-ended (e.g., so as to accept a head or other object to be imaged) while the other end of the structure is "closed" for convenient location of RF coupling and feeding structures. This novel arrangement includes a special perpendicular arrangement of respectively associated RF feedline structures extending across perpendicular diameters of the common closed end. Pairs of coil legs in each of the coil structures are capacitively coupled together at the open end and conductively coupled together at the closed end (where they are conductively coupled to a respectively associated one of the perpendicular feedline structures).
The earlier referenced Fehn et al application discloses a technique for achieving quadrature detection with a pair of surface coils (which, in the exemplary embodiment, may also be center fed in accordance with the present invention).